Engines using gasoline-based fuel typically employ a spark ignition system in which an air-fuel mixture is forcibly ignited using a sparkplug. Meanwhile, in order to improve significantly the fuel economy, it has recently been suggested to cause compressed self-ignition of the air-fuel mixture (pre-mixing compressed self-ignition) by increasing the geometric compression ratio of the engine to a high compression ratio equal to or higher than 15 while still using the gasoline-based fuel.
Patent Document 1 discloses an engine of a spark ignition system which is not of the compressed self-ignition system, but in which the geometric compression ratio is still increased to a high value equal to or higher than 13. This engine uses a pentroof combustion chamber. In this engine, where Si1 denotes cross-sectional areas of the combustion chamber in a plurality of mutually parallel virtual cut planes that are parallel to a linear reciprocating direction of the intake valve and pass through the valve head of the intake valve and Si2 denotes an effective opening area (curtain surface area) between the valve head and the valve seat, the shape of the parts of the combustion engine and the like is set such that a relationship Si1≧Si2 is fulfilled in any virtual cut plane at the center timing of the valve overlap period in which the intake valve and the exhaust valve are both open. As a result, the intake air supplied from the circumferential edge of the valve head into the combustion engine can smoothly pass along the lower surface of the valve head. Therefore, scavenging ability and filling efficiency can be increased.
Patent Document 2 discloses a two-cycle diesel engine in which intake air from an intake valve forms a loop flow along the inner wall surface of a cylinder because the axial line of the intake valve is inclined with respect to the cylinder axis.
In order to perform the compressed self-ignition, it is preferred that as homogeneous an air-fuel mixture as possible be formed inside the cylinder, and for this purpose, it is preferred that a fuel injection valve be disposed in the center of the cylinder and the fuel be uniformly injected from the fuel injection valve into the entire cylinder.
However, it is not easy to form a sufficient amount of a homogeneous air-fuel mixture inside the cylinder of an engine with a very high geometric compression ratio, such as described hereinabove. This is because in the engine with a high geometric compression ratio, a gap between the lower surface of the cylinder head and the piston crown surface is extremely small at the top dead center position of the piston and in the vicinity thereof, and scavenging in the overlap period in which the intake valve and exhaust valve are both open is degraded. In particular, where a cavity is formed in the piston crown surface, scavenging inside the cavity becomes insufficient and a sufficient amount of intake air is difficult to supply into the cavity. This is the major reason why the filling efficiency decreases and therefore the output decreases.    Patent Document 1: Japanese Patent Application Publication No. 2009-162154    Patent Document 2: Japanese Patent Application Publication No. H05-113120